In the fabrication of printed circuit boards using multiple layers, each of the layers is fabricated in face-to-face juxtaposition and has electrically conductive traces thereon which require interconnection between the layers. Such interconnection is usually accomplished by a drilled and then metallic-plated through-hole which extends through the layers and is located to cause the plated through-hole to contact the particular traces or pads thereof on one or more of the layers and create an electrically conductive path between the desired traces on different layers. For such a fabrication technique to be effective, it is very important that each of the inner layers of the printed circuit board be in precise registration or alignment with an exterior layer of the printed circuit board so that the relative positions of the traces on different layers are predictable and the through-holes, which are formed after assembly of the layers, will make contact with the desired traces and only those traces. If an inner layer registration with the exterior layer or with any of the other interior layers by too large of an amount, the through-hole will not contact the desired trace, or will contact an undesired trace, and a defective board will result.
As with most fabrication processes, perfect registration of inner layers with the exterior layer is not possible, and the size and placement of the traces must be designed to take into account the fact that some misregistration is inevitable. The larger the amount of misalignment anticipated, the larger the spaces between traces must be to ensure that the through-hole will contact only the desired traces. This reduces the number of electronic devices which may be placed on the printed circuit board, commonly referred to as the packaging density of the board, and hence increases the cost of the printed circuit boards required for a particular circuit.
In the past, after the printed circuit board was fabricated, it was possible to determine whether or not the layer-to-layer registration was correct or incorrect by simply determining whether or not the through-holes made electrical contact with all of the proper traces and no others. Usually, a particular through-hole was checked to make sure it was in contact with a particular trace, and if it was not, the board failed. On the other hand, if all of the through-holes, or however many that were checked, contacted all of the desired traces, the board passed. Rather than checking all through-holes, a sampling was often made. At other times, a test set of traces and through-holes (called a "test coupon") was created for the purpose of testing registration. In all situations, the test simply produced a "pass" or "no pass" result, without any quantification of the amount of layer misregistration involved.
Of course, the inner layers of the fabricated printed circuit board cannot be viewed to visually determine the amount of misregistration, at least not without destroying the board by cutting it apart. Thus, if a particular board passed the test, there was no guarantee that the board would not later fail due to thermal changes or mechanical flexing which might be sufficient to cause a through-hole to lose electrical contact with a desired trace or make electrical contact with an undesired trace. In other words, one could never be sure that a board which passed the registration test did not have marginally adequate layer-to-layer registration and would not later fail during use. As a result, larger-than-necessary allowances in size and position of traces and spaces therebetween were made to lessen the likelihood that a board that passed a registration test would subsequently fail during use because the layer-to-layer registration was barely within tolerance.
The packaging densities of printed circuit boards have increased dramatically, with more and smaller components requiring increased numbers of interconnecting traces. This requires that the traces be reduced in size and the space between the traces be reduced accordingly. To assure accuracy in fabrication, the photo tooling, image transfer, etching, drilling, lamination and other processes involved in the fabrication of printed circuit boards must be controlled within ever tighter tolerance limits. As the board manufacturers approach the inherent accuracy limitations on these processes, it becomes critical to quantitatively measure the amount of misregistration between layers rather than simply conducting a pass/no-pass registration test. While such knowledge is helpful, not knowing the amount by which the layers are out of registration, it is impossible to determine whether through-holes are making adequate electrical contact with traces or whether minimum electrical clearances are being maintained between through-holes and traces on the hidden inner layers of the printed circuit board.
Further, in those applications where failure costs are unacceptably high, as in printed circuit boards used in life support systems and military equipment, over designing into additional layers and higher costs will not necessarily prevent the recurrent misregistration which results from a failure to identify and cure process failures. In this regard, many boards may be produced which will fail the "pass" or "no pass" test leading to further added expense. Such a system requires not only knowledge of the amount of failure but also the direction of misregistration, that is whether a layer is displaced to the right, left, forward or backwards. Further, such a system requires knowledge of the misregistration of each board regardless of the quantity of misregistration of other boards.
It will therefore be appreciated that there has a been a significant need for a system for measuring the amount and direction of misregistration separately for each printed circuit board layer in quantitative terms. The system should allow the measurements to be conducted quickly, economically, and precisely. Such a system would avoid the current industry practice of overcompensating at the design phase to minimize the impact of inner layer misregistration which prevents the fabrication of printed circuit boards with greater densities. Such a system would also aid in preventing continual production of failing boards due to the inability to identify and correct process failures. The result would be more cost effective production of printed circuit boards, as discussed above.